1. Field of the Invention
The present invention relates to surface-mounted antennas and communication apparatus using the same, such as cellular phones.
2. Description of the Related Art
With the recent expanding proliferation of cellular phones, the technological development race is heating up with the aims of miniaturization and high performance of the cellular phones. In order to accomplish there aims, it is essential to reduce the sizes of antennas incorporated in the cellular phones while improving the performance of the antennas.
Although various antenna structures have been designed and provided, no antenna can sufficiently satisfy the demands for both high performance and miniaturization. This type of antenna is yet to appear on the market.
As a promising compact and highly efficient antenna the present inventors have focused much attention on a surface-mounted antenna. The surface-mounted antenna, for example, can be made by disposing a pair of electrodes close to each other on a surface of a dielectric base chip. In this case, the pair of electrodes is comprised of a radiation electrode of an inverted-F antenna and a radiation electrode of a microstrip antenna. In order to reduce the size of the surface-mounted antenna, the pair of radiation electrodes needs to be disposed on the surface of the dielectric base chip with a narrow gap therebetween.
However, when the pair of radiation electrodes is disposed with a narrow distance therebetween without considering the structure in which the electrodes are disposed, signal interference occurs between the radiation electrodes. This creates a critical problem in that it is difficult for the antenna to have a wide frequency bandwidth. In order to solve the problem, it is only necessary to greatly increase the gap between the pair of electrodes so that the signal interference between the pair of electrodes can be suppressed. However, increasing the gap between the radiation electrodes leads to an increase in the size of the base dielectric substrate, thereby necessarily increasing the size of the surface-mounted antenna. In other words, miniaturizing the antenna and obtaining a wide frequency band for the antenna are mutually contradictory objectives. When either one of these two objectives is achieved, the other one is difficult to achieve. Thus, with such a simple structure in which the pair of electrodes is disposed together, it is unlikely that the demands on miniaturization of the antenna and on obtaining of a wide frequency band for the antenna can be met.
However, by conducting much research and development, the present inventors have successfully designed an antenna having an innovative electrode-arrangement structure, which can meet both demands mentioned above.